Joule-Thomson effect

Also known as the Joule-Kelvin effect. When a gas
expands through a porous plug, a change of temperature
occurs, proportional to the pressure difference
across the plug; the Joule-Thomson coefficient is the change
of temperature per unit change of pressure. The temperature change is due
to a departure of the gas from Joule's laws,
the gas performing internal work in overcoming the mutual attractions of
the molecules and thus cooling itself;
and partly to deviation of the gas from Boyle's
law. The latter effect can give rise either to cooling or heating, depending
upon the initial temperature and pressure difference used. For a given mean
pressure, the temperature at which the two effects balance, resulting in
no alteration of temperature, is called the inversion temperature.
Gases expanding through a porous plug below their inversion temperature
are cooled, otherwise they are heated. See also cryogenics.